KrF excimer lasers are the state of the art light source for integrated circuit lithography. One such laser is described in U.S. Pat. No. 4,959,840 issued Sep. 25, 1990. The lasers operate at wavelengths of about 248 nm. With the KrF laser integrated circuits with dimensions as small as 180 nm can be produced. Finer dimensions can be provided with ArF lasers which operate at about 193 nm or F2 lasers which operate at about 157 nm.
These lasers, the KrF laser, the ArF laser and the F2 lasers, are very similar, in fact the same basic equipment used to make a KrF laser can be used to produce an ArF laser or an F2 laser merely by changing the gas concentration and modifying the controls and instrumentation to accommodate the slightly different wavelength.
Control of lithography lasers and other lithography equipment require laser pulse energy monitors sensitive to the UV light produced by these lasers. The standard prior art detectors used for monitoring pulse energy in state of the art integrated circuit lithography equipment are silicon photo diodes.
A typical prior-art KrF excimer laser used in the production of integrated circuits is depicted in FIG. 1 and FIG. 2. A cross section of the laser chamber of this prior art laser is shown in FIG. 3. As shown in FIG. 2A, pulse power system 2 powered by high voltage power supply 3 provides electrical pulses to electrodes 6 located in a discharge chamber 8. Typical state-of-the art lithography lasers are operated at a pulse rate of about 1000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas) at about 3 atmospheres is circulated through the space between the electrodes at velocities of about 1,000 inches per second. This is done with tangential blower 10 located in the laser discharge chamber. The laser gases are cooled with a heat exchanger 11 also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by line narrowing module 18 (sometimes referred to as a line narrowing package or LNP). Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:
Laser Chamber Module,
Pulse Power System with: high voltage power supply module,
commutator module and high voltage compression head module,
Output Coupler Module,
Line Narrowing Module,
Wavemeter Module,
Computer Control Module,
Gas Control Module,
Cooling Water Module
Electrodes 6 consist of cathode 6A and anode 6B. Anode 6B is supported in this prior art embodiment by anode support bar 44 which is shown in cross section in FIG. 3. Flow is counter-clockwise in this view. One corner and one edge of anode support bar 44 serves as a guide vane to force air from blower 10 to flow between electrodes 6A and 6B. Other guide vanes in this prior art laser are shown at 46, 48 and 50. Perforated current return plate 52 helps ground anode 6B to the metal structure of chamber 8. The plate is perforated with large holes (not shown in FIG. 3) located in the laser gas flow path so that the current return plate does not substantially affect the gas flow. A peaking capacitor comprised of an array of individual capacitors 19 is charged prior to each pulse by pulse power system 2. During the voltage buildup on the peaking capacitor, two preionizers 56 weakly ionize the lasing gas between electrodes 6A and 6B and as the charge on capacitors 19 reach about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse. Following each pulse, the gas flow between the electrodes of about 1 inch per millisecond, created by blower 10, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring one millisecond later.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of the subsequent pulse is close to the desired energy.
These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance. One problem experienced with these prior-art lasers has been excessive wear and occasional failure of blower bearings. A need exists in the integrated circuit industry for a modular, reliable, production line quality F2 laser in order to permit integrated circuit resolution not available with KrF and ArF lasers.
A well-known technique for reducing the band-width of gas discharge laser systems (including excimer laser systems) involves the injection of a narrow band xe2x80x9cseedxe2x80x9d beam into a gain medium. In one such system, a laser producing the seed beam called a xe2x80x9cmaster oscillatorxe2x80x9d is designed to provide a very narrow band beam and that beam is used as a seed beam in the second laser. If the second laser functions as a power amplifier, the system is referred to as a master oscillator, power amplifier (MOPA) system. If the second laser itself has a resonance cavity, the system is referred to as an injection seeded oscillator (ISO) and the seed laser is called the master oscillator and the downstream laser is called the power oscillator. These techniques reduced the heat load on the line narrowing optics.
Laser systems comprised of two separate lasers tend to be substantially more expensive, larger and more complicated than comparable single laser systems. Therefore, commercial applications of two laser systems has been limited. In most examples of prior art MOPA and ISO systems two separate laser chambers are utilized. However, systems have been proposed for using a single laser chamber to contain two sets of electrodes. For example, FIG. 3A shows a side-by-side arrangement described by Letardi in U.S. Pat. No. 5,070,513. Another arrangement shown in FIG. 3B described by Long in U.S. Pat. No. 4,534,035 in which the elongated electrode sets are positioned on opposite sides of the chamber. Gas flows from a common xe2x80x9cinxe2x80x9d plentum separately between the two sets of electrodes into a common xe2x80x9coutxe2x80x9d plenum. An arrangement proposed by McKee in U.S. Pat. No. 4,417,342 is shown in FIG. 3C. This system has two elongated electrode sets mounted parallel to each other on one half of the chamber. A tangential fan and heat exchanger is located in the other half. Gas flows in parallel through between the two sets of electrodes. The system shown in FIG. 3A has not been considered suitable for high pulse rate laser because debris from the upstream discharge interferes with the downstream discharge. According to an article published in Applied Physics B Lasers and Optics 1998, this laser is operated at a pulse repetition rate of about 100 pulses per second. The authors indicate that an attempt to operate at 1000 Hz would lead to turbulent flow which is not desirable for generation of a high quality beam. The system shown in FIG. 3C has not been considered suitable for high pulse rate lasers because splitting of the flow reduces the velocity of the gas between the electrodes by about 50% as compared to a single set of electrodes on the system shown in FIG. 3A. The system shown in FIG. 3B has not been considered satisfactory for high pulse rate lasers because the blower circulation is axial rather than tangential as shown in FIG. 3.
A typical KrF laser has a natural bandwidth of about 400 pm (FWHM) centered at about 248 nm and for lithography use it is line narrowed to about 0.6 pm. ArF lasers have a natural bandwidth of about 40 pm centered at about 193 nm and is typically line narrowed to about 0.5 pm. These lasers can be relatively easy tuned over a larger portion of their natural bandwidth using the line narrowing module 18 shown in FIG. 2. F2 lasers typically produce laser beams at two narrow lines centered at about 157.63 and 157.52. Often, the less intense of these two lines is suppressed and the laser is forced to operate at the 157.63 line. The natural bandwidth of the 157.63 line is about 1.0 to 1.6 pm. A problem with the F2 laser and this line for lithography purposes is that the line is not narrow enough to satisfy focusing requirements and it is too narrow to provide desired tuning flexibility.
What is needed is an improved narrow band F2 laser system.
The present invention provides a tunable injection seeded very narrow band F2 lithography laser. The laser combines modular design features of prior art long life reliable lithography lasers with special techniques to produce a seed beam operated in a first gain medium which beam is used to stimulate narrow band lasing in a second gain medium to produce a very narrow band laser beam useful for integrated circuit lithography. In a preferred embodiment, two tunable etalon output couplers are used to narrow band an F2 laser and the output of the seed laser is amplified in an F2 amplifier.